Paper and paperboard are often externally sized with various materials for the purpose of increasing their strength, their resistance to picking and scuffing, and their resistance to undue penetration of water, organic solvents, oils, inks and various types of aqueous solutions as well as for the purpose of improving their smoothness and optical characteristics. When sizing materials are applied to the surface of a web or sheet in order to cement the surface fibers to the body of the paper and to modify the sheet surface, the process is known as external or surface sizing; the latter process being quite distinct from an internal sizing process wherein sizing agents are admixed with the pulp slurry prior to its being converted into web or sheet form, to reduce penetration of aqueous and other fluid into the paper.
Among various materials which have been utilized as external sizing agents are included conventional and modified starches, polyvinyl alcohol, cellulosic derivatives, gelatin, rosin, proteins such as casein, natural gums and synthetic polymers. Although these materials are effective to various degrees under certain conditions, their use is nonetheless subject to one or more limitations. For example, it is often necessary to utilize high concentration levels of such sizes in order to achieve the desired properties. Since it is known that the opacity and brightness of the base paper sheet decrease in proportion to the amount of size that is applied thereto, a direct result of the required use of such high concentration levels is a reduction in the optical properties of the treated paper. Furthermore, the use of such high concentration levels makes the sizing of specialty papers economically unattractive due to the high cost of quality sizes, e.g. specialty starches as well as other natural and/or synthetic polymers which are usually utilized for such purposes. In addition, certain sizing agents impart relatively poor water resistance and must be used in conjunction with insolubilizing agents to assure that satisfactory water resistance is obtained.
The use of hydrophobic starches as external sizing agents has been proposed (see e.g. U.S. Pat. No. 2,661,349 to Caldwell et al.), but the use of such agents is impractical due to their high viscosity; starches must be partially fluidized (degraded), prior to use, adding to their cost. To remedy this, Gaspar et al. (in U.S. Pat. No. 4,239,592, incorporated herein by reference) proposed using blends of non-hydrophobic and hydrophobic starches as external sizes. While such starch blends are used, the utility of such blends is limited, since, according to Gaspar, any increase in the amount of hydrophobic starch in the blend beyond 14% will not result in increased sizing properties. Because the degree of sizing is directly proportional to the amount of hydrophobic starch in the blend, the utility of the blends is limited to applications where the degree of sizing required is not high.
There exists a need for sizing compositions which do not exhibit these drawbacks, are relatively inexpensive, are easy to prepare, and impart desirable properties to the paper or paperboard substrate.